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.\"     @(#)write.2	8.5 (Berkeley) 4/2/94
.\"
.Dd April 3, 2010
.Dt WRITE 2
.Os
.Sh NAME
.Nm write ,
.Nm writev ,
.Nm pwrite ,
.Nm pwritev
.Nd write output
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft ssize_t
.Fn write "int d" "const void *buf" "size_t nbytes"
.Ft ssize_t
.Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset"
.In sys/uio.h
.Ft ssize_t
.Fn writev "int d" "const struct iovec *iov" "int iovcnt"
.Ft ssize_t
.Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
.Sh DESCRIPTION
.Fn write
attempts to write
.Fa nbytes
of data to the object referenced by the descriptor
.Fa d
from the buffer pointed to by
.Fa buf .
.Fn writev
performs the same action, but gathers the output data
from the
.Fa iovcnt
buffers specified by the members of the
.Fa iov
array: iov[0], iov[1], ..., iov[iovcnt\|-\|1].
.Fn pwrite
and
.Fn pwritev
perform the same functions, but write to the specified position in
the file without modifying the file pointer.
.Pp
For
.Fn writev
and
.Fn pwritev ,
the
.Fa iovec
structure is defined as:
.Pp
.Bd -literal -offset indent -compact
struct iovec {
	void *iov_base;
	size_t iov_len;
};
.Ed
.Pp
Each
.Fa iovec
entry specifies the base address and length of an area
in memory from which data should be written.
.Fn writev
and
.Fn pwritev
will always write a complete area before proceeding
to the next.
.Pp
On objects capable of seeking, the
.Fn write
starts at a position
given by the pointer associated with
.Fa d
(see
.Xr lseek 2 ) .
Upon return from
.Fn write ,
the pointer is incremented by the number of bytes which were written.
.Pp
Objects that are not capable of seeking always write from the current
position.
The value of the pointer associated with such an object
is undefined.
.Pp
If the real user is not the super-user, then
.Fn write
clears the set-user-id bit on a file.
This prevents penetration of system security
by a user who
.Dq captures
a writable set-user-id file
owned by the super-user.
.Pp
If
.Fn write
succeeds it will update the st_ctime and st_mtime fields of the file's
meta-data (see
.Xr stat 2 ) .
.Pp
When using non-blocking I/O on objects such as sockets that are subject
to flow control,
.Fn write
and
.Fn writev
may write fewer bytes than requested;
the return value must be noted,
and the remainder of the operation should be retried when possible.
.Sh RETURN VALUES
Upon successful completion the number of bytes which were written
is returned.
Otherwise a \-1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn write ,
.Fn writev ,
.Fn pwrite ,
and
.Fn pwritev
will fail and the file pointer will remain unchanged if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The file was marked for non-blocking I/O,
and no data could be written immediately.
.It Bq Er EBADF
.Fa d
is not a valid descriptor open for writing.
.It Bq Er EDQUOT
The user's quota of disk blocks on the file system
containing the file has been exhausted.
.It Bq Er EFAULT
Part of
.Fa iov
or data to be written to the file
points outside the process's allocated address space.
.It Bq Er EFBIG
An attempt was made to write a file that exceeds the process's
file size limit or the maximum file size.
.It Bq Er EINTR
A signal was received before any data could be written to a slow
device.
See
.Xr sigaction 2
for more information on the interaction between signals and system
calls.
.It Bq Er EINVAL
The pointer associated with
.Fa d
was negative; or
the total length of the I/O is more than can be expressed by the ssize_t
return value.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er ENOSPC
There is no free space remaining on the file system
containing the file.
.It Bq Er EPIPE
An attempt is made to write to a pipe that is not open
for reading by any process; or
an attempt is made to write to a socket of type
.Dv SOCK_STREAM
that is not connected to a peer socket.
.El
.Pp
In addition,
.Fn writev
and
.Fn pwritev
may return one of the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa iovcnt
was less than or equal to 0, or greater than
.Brq Dv IOV_MAX ;
or one of the
.Fa iov_len
values in the
.Fa iov
array was negative; or
the sum of the
.Fa iov_len
values in the
.Fa iov
array overflowed a 32-bit integer.
.El
.Pp
The
.Fn pwrite
and
.Fn pwritev
calls may also return the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The specified file offset is invalid.
.It Bq Er ESPIPE
The file descriptor is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr lseek 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr poll 2 ,
.Xr select 2 ,
.Xr sigaction 2
.Sh STANDARDS
The
.Fn write
function is expected to conform to
.St -p1003.1-88 .
The
.Fn writev
and
.Fn pwrite
functions conform to
.St -xpg4.2 .
.Sh HISTORY
The
.Fn pwritev
function call
appeared in
.Nx 1.4 .
The
.Fn pwrite
function call
appeared in
.At V.4 .
The
.Fn writev
function call
appeared in
.Bx 4.2 .
The
.Fn write
function call appeared in
.At v2 .
.Sh CAVEATS
Error checks should explicitly test for \-1.
Code such as
.Bd -literal
	while ((nr = write(fd, buf, sizeof(buf))) \*[Gt] 0)
.Ed
.Pp
is not maximally portable, as some platforms allow for
.Va nbytes
to range between
.Dv SSIZE_MAX
and
.Dv SIZE_MAX
\- 2, in which case the return value of an error-free
.Fn write
may appear as a negative number distinct from \-1.
Proper loops should use
.Bd -literal
	while ((nr = write(fd, buf, sizeof(buf))) != -1 \*[Am]\*[Am] nr != 0)
.Ed
